The solid contents influences highly the processability of such metals and the quality of the final products. It is known that there is a relationship between the melting temperature applied and the eventual solid contents of the slugs. However, the temperature of the hot slugs is not easy to determine. Moreover, the ranges within which a thixotropic behaviour occurs are relatively narrow, thus demanding a high accuracy in measuring temperatures.
In the German Journal "Giesserei", 80 (1993), No. 4/February 22, pp. 111 and 112, an arrangement is described wherein a measuring signal being in direct relationship to the solid contents is obtained by a magnet situated near the lower front surface of the slug and measuring an eddy current.
In fact, this reference is a report about an experimental arrangement at the Technical University of Brunswick (Germany) rather than a field test out of the practice. For to practice, this arrangement is hot suited for several reasons, as tests of the inventors have proved.
First, the lower front surface of a slug, in general, is just that onto which the slug shall stand, because otherwise difficulties will result to hold it. If this surface is contacted with a supporting surface, there is a heat exchange whereby just the surface whose temperature is measured has not that solid contents which is contained in the upper part of the slug. Furthermore, there can be differences in solid contents in radial direction due to the progress in heating up the slug from the exterior to the interior. A further factor may consist in possible interferences of the eddy currents measured by the inductive heating coil, although an arrangement of the sensor at the front surface involves a certain distance from this heating coil.
Therefore, it is a drawback of this, in itself, good idea that the measuring results are not reliable in all cases due to various factors to which bad conditions for handling the slug will add, if its lower front surface shall be used for measuring.